Sound radiator



July 25, 1933. C, D HMGIS 1,919,632

SOUND RADIATOR Filed April 4, 1930 1 nM-9 1 Munn-9, 3 9

Patented July 25, 1933 UNITED STATES PATENT OFFECE y 'C'ARLETON D.HAIGIS, 0F HADDNFIELD, NEW' JERSEY, ASSIGNOR, BY MESNE ASSIGN- MENTS, IORADIO CORPORATION F .ff-LIRIERIC., 07? NET YORK, N. Y., A CQEPORATION OFDELAWARE SOUND RDIAT 0R Application aree April 4, 1930.

This invention relates to acoustic devices, and more particularly tosound radiators of the large directacting type.

f s It has heretofore been proposed to provide sound radiators or loudspeakers with large direct-acting d iaphragms, having an area for soundradiation of several square feet, where'large volumes of sound are de--sired, as in auditoriums and the like, but in all such devices asheretofore employed, so far as I am aware, it has been proposed toemploy diaphragm material which has low internal viscosity. The rate ofpropagation of sound in a diaphragm is proportional to the square rootof the tension divided by the mass, and in order to minimize thesett-ing up of standing Waves in the diaphragm material, with consequentdisadvantageous resonances and interferences which are productive ofnon-uniform sound propagation, the diaphragm material has been subjectedto high tension in the direction of its planethe desideratum being to sotension the dia* yphragm material that the time required for the soundto travel to the periphery of the diaphragmfrom the point or points atwhich the driving forceis applied thereto is less than the timerequiredfor the propagated wave to travel half a wave length in the airat right angles to the plane of the diaphragm, so as to minimize thephase difference between the center and periphery of a wave frontradiated from the diaphragm. The

desired goal can be approached by increasing the tension and decreasingthe mass of the diaphragm, but experience has demonstrated that thereare no known materials which are both strong enough to withstand thenecessary tension and light enough to have the desired mass toaccomplish successfully the end desired. Therefore devices of the typeunder consideration as heretofore proposed have been open to theobjection that within a certain range of the wave lengths reproducedstanding waves are set up in the dia phragm material, and interferences,phase distortion and resonances are produced, with a consequentnon-uniformity of sound reproduction. v

Itis an obj ect of this invention to provide Serial No. 441,559.

a large direct-acting sound radiator' which overcomes the aboveobjections and which is capable ot high quality reproduction of musicand speech without .the interferences, phase distortions and resonancesthat have heretofore characterized devices of this type.

Another object of this invention is to provide a large direct-actingsound radiator which employs a diaphragm of high internal viscosity soas to minimize the setting up of standing waves in the diapragm materialand the consequent non-uniformity of sound reproduction.

Another object of this invention is to provide a relatively largedirect-acting sound radiator of the type referred to in combination witha novel driving unit of rela-tively high efficiency which isparticularly adapted to the actuation of such a sound radiator.

Another object of this invention is to provide a driving unit which iscoupled with a source of audio frequency current wherein the impedanceof the source is matched with the tot-al impedance of the radiator,including the electrical resistance and reactance of mechanical reactionas well as the electrical constants of driving unit, so as to obtain amaximum transfer of energy and power and con sequent high efiiciency.

Other objects will appear as the description of the invention proceeds.

The invention is capable of receiving a variety of mechanicalexpressions, one of which is shown on the accompanying drawing, but itis to be expressly understood that the drawing is for purposes ofillustration only, and is not to be construed as a definition of thelimits of the invention, reference being had to the appended claims forthat purpose..

In the drawing, wherein like reference characters are employed todesignate corresponding parts in the several views Fig. l is an endelevation of an embodiment of the present invention;

Fig. 2 is a side elevation oit the embodiment of Fig. l;

Fig. 3 is a perspective view illustrating the improved driving unit inconjunction with a diaphragm embodying the present invention removedfrom its supporting frame; and

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Fig. -t is a fragmentary view to illustrate how the mounting means maybe attached to the diaphragm.

In accordance with the present invention the diaphragm is composed of amaterial having a high internal viscosity, so that after a Wave hastraveled through the. diaphragm material for a distance of approximatelyhalf' a Wave length from the point or line at which the driving force isapplied thereto, the energy of the wave is largely absorbed, and hencestanding waves, due to reiiectien Jfrom the edges of tho diaphragm, arelargely, if not completely, suppressed by absorption in the material,due to its high internal friction. This means that only a portion of thediaphragm around or on either side of the point or line at which thedriving force is applied thereto is used as a sound radiator, the activearea of the diaphragm for any given frequency varying inversely to thefrequency.

Any suitable material of high internal viscosity may be employed, lVood,of sufficiently strong texture, such as pine, or laminated wood andfabric as canvas. as well as certain impregnated canvas or othersuitable fabrics provided as substitutes for veneering, sheathing, ctc.,have been found to give excellent results. For example, heav i clothimpregnated With a phenol-resin or the like, such as the material knowncommercially as Celeron is very satisfactory. lith such material thecentral area of the diaphragm, depending upon the particular frequencybeing radiated, becomes the sound generating surface, and beyond an arearepresented by approximately half a wave length from the point or lineof application of the driving force to the diaphragm, the energy of theWave is largely absorbed by the internal friction of the material, sothat the setting up of standing waves is minimized and the consequentresonances, phase distortions and interferences heretofore existing byreason of standing waves, and the consequent non-uniformity of soundradiation, are largely eliminated.

The diaphragm may be of any suitable size. For example, a diaphragmhaving free area kon the order of six feet by three feet has been foundto give excellent results, but if preferred narrower or greater widthsor greater or shorter lengths, or both, may be employed. The diaphagmmay also be of any suitable thickness depending in part on the characterof material employed and the size used.

In order to normally maintain the diaphragm in its neutral position, arelatively light tension is applied thereto in the direction of itsplane, so that the diaphragm properly restored to its neutral orintermediate position whenever deflected therefrom by the driving forceapplied thereto, but the restoring tension thus applied to the diaphragmis to be sharply differentiated from the tension heretofore employed onlarge diaphragms for he purpose of so tensioning thc diaphragm materialthat the rate of vibration propagation in the diaphragm material shallbe large as compared with the rate of vibration propagation in the air.In fact, the amount of tension applied to the diaphragm in accordancewith the present invention is not of great importance and can be variedthrough conf-iiderable limits without affecting the action of thediaphragm. Hence, it is onlyT at the relatively low frcduenf under a'thousand cycle-f, where the rela onship first above discussed ceases,that the tension applied to the diaphragm has any substantial ellect onits action as a sound radiator.

rhe diaphragm embodying the present invention may be mounted in anysuitable way. As shown, the diaphragm 10 is supported in a fran'ieworkof any suitable construction, shown as composed of angularly related endframe pieces il suitably braced by transverse pieces l2 and connected atthe top and bottom by longitiulinals 13 and 14:. In Figs. 1 to 3 thediaphragm is shown as provided at its top and bottom edges with bracingstrips l5 and i6, and at its lower edge the diaphragm is fixedly securedto the longitudinal 14 by a plurality of suitably spaced clips orclamping members 17. At its upper edge he diaphragm is resilientlysupported so that a moderate tension may be applied thereto in thedirection et the plane. Any suitable tension support may be employed,that shown comprising a plurality of hails 18 suitably attached to andspaced along the upper bracing strips l5 and into which are engagedhooks 19 carried by coiled tension springs 20. which at their oppositeends are connected through rings 2l to threaded adjusting bolts 22suitably threaded into the upper longitudinal i3 of the frame or nutscarried thereby, hand wheels 23 being shown as applied to each of saidbolts for the purpose of. varyingl or apililying the desired degree oftension of said springs 20.

In place of the bracing strips Yl5 and 16, the diaphragm as shown inFig. 4t may have secured thereto along each of its upper and lower edgesand at eacl side thereof a plurality of lozengc shaped elements l5 ofsuitable material, Bakelite, suitably attached thereto, as by rivets,each of said members having a. central aperture through which theattaching means for the tension springs and the lower mounting clips orclamps may be applied.

One or more driving units of any suitable type or construction aremounted ou the frame in any suitable ,vay and suitably attaehed to thediaphragm, preferably along a line approximatelv midway between itsupper and lower ed@ As shown, a platform 9.4 is suitably car on theframe by cleats 25 and struts 26. A plurality of electro--magneticdriving units `may be mounted on the platform 24; andattached tothe dia-` phragm Vl0 in any suitable'way at 27.V

I prefer to operate vthe diaphragm however by a novel driving unit ofthe character illustrated in Fig.' 3 and which extends substantially theentire `width of theV diaphragm 10. As here shown, the driving unitcomprisesafieldfcoil 28 and a magnet core 29 of substantially the widthof the diaphragm, said core having an air gap30. Disposed in the air gap30 isA a conductor for audio frequency currents-in the form of a strip3l, preferably of relatively light material, which is suitably attachedto the diaphragm, as by a plurality Vof extensions 32 which extendthrough apertures disposed along the width of the diaphragm atapproximately its niid length, and which extensions are suitably securedto the diaphragm rigidly as by nuts 33 threaded onto said extensions atopposite sides `of the diaphragm. Said strip 3l may be made of aluminumor other light metal or alloy and may be stamped or pressed from asingle sheet of metaka strip en the order of a quarter of an inch inWidth and a sixteenth of an inch in thickness having beenfound'suitable. The extensions for attachment to the diaphragm should beof small `dimension so as to prevent deviation of the audio current fromthe main path, and they should be spaced relatively close together sothat a driving force may be generated uniformly along the strip for allfrequencies likely to be transmitted, and prevent the ilexure of thestrip between the extensions and consequent resonance of the strip atthe higher frequencies.

The powerful electromagnet 28, 29 provides a high flux density when highgrade iron is used, a density of 15,000 lines per square inch beingavailable in the gap, and when audio frequency current passes throughthe strip 3l across the path of the magnetic flux, a force is generatedtending to move the strip 3l at right angles `to both the current andthe iux in accordance with Lenz law. Thereby the strip, and thediaphragm attached thereto, are moved outwardly or inwardly with respectto the gap, depending upon the instantaneous direction of the audiofrequency current. The electrical impedance measured between the ends ofthe strip 3l is relatively low, being composed of the true electricalresistance and reactance of the strip itself, together with theelectrical constants generated in the strip by the mechanical reactionof the diaphragm thereon. As' the electrical resistance and reactance,due to the mechanical reaction, are much greater than those of the stripitself, the maximum transfer of energyv er power requires that theimpedance of the source be matched with the impedance of the speaker.Where the total impedance is very low a special output transformershould be cmployed for transforming the high impedance of the vacuumtube plate circuit employed as a source to alow impedance to match thespeaker. l."ilith a. driving unit so constructed high eiliciencyobtained in the actuation of thediaphragm and the sound produced,whether of music or speech, is found to be of high quality.

'It will. therefore be perceived that a large direct-acting soundradiator has been provided which avoids the difficulties heretoforeexperienced in producing uniform sound radiation with diaphragms of thecharactcr under consideration, and which enables a diaphragm of severalsquare feet in area to be employed without setting up standing waves inthe diaphragm material or intro" duciiml the resonances, distortions andinterferences that have heretofore been characteristic of diaphragms ofthis character.

Moreover, a novel driving unit has been provided which operates at highellieiency and enables improved action of the speaker a whole.

lhile the illustrated embodiment of the invention has been describedwith considerable particularity, it to be expressly understood that theinvention is not limited thereto, as other embodiment thereof will nowreadily suggest themselves to those skilled in the art, while changesmay be made in the details of construction, arrangement, size andproportion of parts and material einl'iloyed without departing from thespirit of this invention. A. diaphragm embodying thepresent inventionmay be used with other driving` units, and the driving unit of thepresent invention may be used in conjunction with other diaphragms.although it has been shown that the combination of the two isproductiveof highly desirable results and improved efficiency in the radiation ofhigh quality speech and music. Also the shape of the diapliragm may bechanged and the point or line of application of the driving forcethereto may be varied within the contemplation of the invention. Thedirection of application of the tension springs may also be changed, itbeing inimaterial whether the springs pull vertically, horizontally orat an angle.

Reference is tlierefore to be had to the appended claims for adefinition of the limits of the invention.

That is claimed is l. An acoustic device including a relatively largedirect-acting thin sheet diaphragm of high internal viscosity and whosearea is on the order of a plurality of square feet, a frame in whichsaid diaphragm is mounted, means for rigidly attaching one end of saiddiaphragm to said frame, a plurality of flexibly mounted free tensionsprings for applying a restoring tension to the opposite edge of saidCFI diaphragm without imposing an audio tension on the material thereof,a single high frequency responsive means operatively connected to saiddiaphragm substantially midway of its length for vibrating the same,said last named means including aV single thin strip of light weightcurrent conducting material rigidly secured to the diaphragm in a planesubstantially at a right angle to the plane of the diaphragm and to thedirection of tension in the diaphragm, and means for electricallyenergizing said. strip of material uniformly throughout its length,whereby the diaphragm is caused to move directly by the strip ofconducting material by electrical forces applied thereto.

2. In an acoustic device, iii combination with a relatively largedirect-acting sheet diaphragm, a single light Weight conductor adaptedfreely to respond to audio frequency impulses, extending substantiallythe full width of the diaphragm and being rigidly attached thereto in asinglestraight line along its width and substantially midway between itsends, and a single means providing an electrical field for saidconductor substantiaL ly throughout its length, said diaphragm having anarea and an internal viscosity such that only a central area thereof isresponsive for the generation of sound.

3. In an acoustic device of the character described, a sheet ofphenolic-resin impregnated fabric providing a diaphragm, means connectedwith opposite edges of said diaphragm for resiliently mounting the sameunder tension in the direction of said edges, the remaining edges of thediaphragm being free, an audio frequency driving unit for said diaphragmincluding a single thin strip of light weight current conductingmaterial rigidly secured to the diaphragm in a plane substantially at aright angle to the plane of the diaphragm and to the direction oftension in the diaphragm, said means being located substantially midwaybetween the suppoited edges of the diaphragm, and means for electricallyenergizing said strip of conducting material whereby throughout itslength it imparts motion uniformly directly to the diaphragm along theline of attachment thereto, said diaphragm having an area and aninternal viscosity of material such that the sound generating surfacethereof is limited to a central area adjacent to said conducting strip.4

`CARLETON D. HAIGIS.

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